The present invention relates to a planar lightwave circuit and, more particularly, to a planar lightwave circuit having regions sandwiched between a plurality of waveguides like a star or Y-branching waveguide.
Conventionally, a planar lightwave circuit formed on a planar substrate can have various functions such as multiplexing/demultiplexing, optical branching, and optical switching, and hence is expected as a practical optical device or component. A multi/demultiplexer and optical branching circuit, in particular, are expected as passive parts important for a wavelength multiplexing network system and access network.
FIGS. 8 to 10 show an arrayed-waveguide grating multi/demultiplexer using silica glass for a planar lightwave circuit. FIG. 8 shows the arrayed-waveguide grating multi/demultiplexer. FIG. 9 shows part of the arrayed-waveguide grating multi/demultiplexer. FIG. 10 shows part of a cross section taken along a line B-B' in FIG. 9.
As shown in FIG. 8, in this arrayed-waveguide grating multi/demultiplexer, first of all, signal light incident from input waveguides 801 is expanded in an input-side slab waveguide 802 and strikes an arrayed waveguide 803. In the arrayed waveguide 803, since optical path length differences are set between the adjacent waveguides, the signal light which is guided through the arrayed waveguide 803 and incident on an output-side slab waveguide 804 has phase differences. The signal light is therefore focused and demultiplexed by different output waveguides 805 depending on the wavelengths satisfying diffraction conditions.
In the arrayed waveguide 803, as shown in FIGS. 9 and 10, cores 803a are clearly separated from each other. In the connection portion between the arrayed waveguide 803 and the input-side slab waveguide 802 or output-side slab waveguide 804, spacings on the .mu.m order are formed between the cores 803a. As shown in FIG. 10, each core 803a is sandwiched between lower and upper clads 806 and 807 made of silica glass having a refractive index lower than that of the core 803a, thereby forming an optical waveguide.
As described above, a multi/demultiplexer and optical branching circuit are expected as passive components important for a wavelength multiplexing network system and access network. It is essential for these passive components that the propagation loss of light signals is as low as possible.
In the conventional arrayed-waveguide grating multi/demultiplexer shown in FIG. 8, however, there are spacings on the .mu.m order are formed between the respective cores 803a at the connection region between the cores 803a constituting the arrayed waveguide 803 and the input-side slab waveguide 802. For this reason, part of incident signal light from the input-side slab waveguide 802 to the arrayed waveguide 803 is scattered through the spacings of the .mu.m order. The propagation loss of signal light due to this scattering is as large as 50% of the total loss.
As described above, in a conventional circuit in which signal light branches, such as an arrayed-waveguide grating multi/demultiplexer, signal light is scattered through the spacings between the branching cores. Hence, a propagation loss occurs.
According to a reference (C. van Dam, A.A.M. Staring et al., "Loss reduction for phased-array demultiplexers using a double etch technique" Integrated Photonics Research 1996 Boston, Mass., April 29-May 2, pp. 52-55), in an InGaAsP-based arrayed-waveguide grating multi/demultiplexer, a transition region is formed on the boundary between a slab waveguide and an arrayed waveguide by etching halves of cores so as to reduce the propagation loss of signal light. Even if, however, this structure is applied to glass-based waveguides, the propagation loss reducing effect is very small.
In addition, according to this technique, in a lithography process of transferring a circuit pattern, etching must be performed twice after a mask is accurately aligned, resulting in a complicated process.
According to another reference (Jerry C. Chen and C Dragone, "A Proposed Design for Ultralow-Loss Waveguide Grating Routers", IEEE Photon. Technol. Lett., vol. 10, pp. 379-381, March, 1998), a simulation result is reported, which indicates that a reduction in loss can be attained by optimizing a circuit configuration. However, the above problem of scattering of signal light still remains unsolved.
As described above, when signal light is to be branched or demultiplexed from one waveguide or slab waveguide into a plurality of waveguides, the spacings between the respective waveguides at the branching point are ideally 0 in terms of the loss of light.
However, photolithography and etching techniques used in the process of forming waveguides have their own limits of resolution, and the spacings between the respective waveguides (cores), e.g., glass-based waveguides, at the branching point are about 1 .mu.m or more. For this reason, in a conventional planar lightwave circuit, an excess waveguide loss occurs at such a branching portion or demultiplexing portion. Demands have therefore arisen for a reduction in loss at the portion.